>>527889875
Noelle talks about noticing that Kris has changed "recently". The cage and blood stain already existed at the start of the game so it's not possible that possession started when the game started, but it likely was only briefly before that.
If Kris didn't sound "weak and shaky" as a child, then he probably was able to sustain himself - most likely by having his own soul. At some point something probably happened to lose or displace it, potentially related to the dark worlds and his arrangement with the knight.